Time-to-voltage converter using correlated double sampling

1. An apparatus comprising:

a time-to-voltage converter configured to generate an output voltage signal and a correlated reference voltage signal,

wherein the time-to-voltage converter comprises:

a current source configured to generate a bias current through a current source output node;

a first switched-capacitor circuit coupled to the current source output node and configured to generate the output voltage signal based on an input time signal and the bias current during a first interval; and

a second switched-capacitor circuit coupled to the current source output node and configured to generate the correlated reference voltage signal based on a reference time signal and the bias current during a second interval, the first interval and the second interval being non-overlapping intervals.

2. The apparatus, as recited in claim 1 , wherein the second switched-capacitor circuit comprises:

a reference capacitor configured to sample the current source output node to develop a reference voltage during the second interval; and

a hold capacitor selectively coupled in parallel with the reference capacitor and configured to hold the reference voltage outside the second interval.

3. The apparatus, as recited in claim 1 , further comprising:

a third switched-capacitor circuit configured to generate a second correlated reference voltage signal based on a second reference time signal and the bias current during a third interval, the third interval and the second interval being non-overlapping intervals; and

a differential pair of reference voltage terminals, a positive terminal of the differential pair of reference voltage terminals coupled to receive the correlated reference voltage signal and a negative terminal of the differential pair of reference voltage terminals coupled to receive the second correlated reference voltage signal to provide transient voltage neutralization in a circuit coupled to the differential pair of reference voltage terminals.

4. The apparatus, as recited in claim 1 , wherein the output voltage signal has a selectable slew rate.

5. The apparatus, as recited in claim 1 , wherein the current source comprises:

a switched-capacitor circuit; and

a voltage-to-current converter configured to generate an output current based on a reference voltage signal and a feedback signal.

6. The apparatus, as recited in claim 5 , wherein the current source further comprises:

a switched-capacitor resistor configured to vary the bias current as a function of an input period, the switched-capacitor resistor being coupled to a first node, the feedback signal being based on a current through the first node.

7. The apparatus, as recited in claim 5 , wherein the feedback signal is the correlated reference voltage signal.

8. The apparatus, as recited in claim 1 , wherein the first switched-capacitor circuit comprises:

a voltage-to-current converter configured to generate an output current based on a reference voltage signal and a feedback signal.

9. The apparatus, as recited in claim 1 , wherein the time-to-voltage converter is configured as part of a first order sigma-delta modulator-based time-to-digital converter and the first switched-capacitor circuit includes a capacitor that stores a residue charge.

10. The apparatus, as recited in claim 1 , further comprising:

a phase detector configured to generate the input time signal indicative of an input interval; and

an analog-to-digital converter configured to generate a digital code corresponding to the output voltage signal using the correlated reference voltage signal,

wherein the phase detector, the time-to-voltage converter, and the analog-to-digital converter are configured as a time-to-digital converter.

11. The apparatus, as recited in claim 1 , further comprising:

a digital-to-analog converter configured to use the correlated reference voltage signal to generate the input time signal in response to an input digital code and the reference time signal; and

a comparator configured to generate an output time signal in response to a comparison of the output voltage signal and a threshold voltage level of the comparator,

wherein the time-to-voltage converter, the digital-to-analog converter, and the comparator are configured as a digital-to-time converter.

12. A method comprising:

generating a bias current;

generating an output voltage signal that is linearly related to an input time signal based on the bias current during a first interval; and

generating a correlated reference voltage signal, generation of the correlated reference voltage signal comprising:

sampling the bias current during a second interval to generate a bias current sample; and

updating the correlated reference voltage signal using the bias current sample to generate the correlated reference voltage signal,

wherein the first interval and the second interval are non-overlapping periodic intervals of a periodic signal.

13. The method, as recited in claim 12 , further comprising:

converting an input interval between a time representation and a digital signal representation based on the output voltage signal and the correlated reference voltage signal.

14. The method, as recited in claim 12 , further comprising:

generating a second correlated reference voltage signal, generation of the second correlated reference voltage signal comprising:

sampling the bias current during a third interval of the periodic signal to generate the second correlated reference voltage signal; and

holding the second correlated reference voltage signal outside the third interval.

15. The method, as recited in claim 12 , further comprising:

generating a second correlated reference voltage signal that is linearly related to a second reference time signal based on the bias current; and

neutralizing transient voltages in a circuit by providing the correlated reference voltage signal and the second correlated reference voltage signal to a differential pair of reference voltage terminals of the circuit, a positive terminal of the differential pair of reference voltage terminals coupled to receive the correlated reference voltage signal and a negative terminal of the differential pair of reference voltage terminals coupled to receive the second correlated reference voltage signal.

16. The method, as recited in claim 12 , wherein generating the bias current comprises:

converting a reference voltage to an output current based on a feedback signal.

17. The method, as recited in claim 16 , wherein generating the bias current further comprises:

varying the bias current and the feedback signal as a function of an input period.

18. The method, as recited in claim 16 , wherein the feedback signal is the correlated reference voltage signal.

19. The method, as recited in claim 12 , wherein generating the correlated reference voltage signal further comprises:

generating an output current based on a reference voltage signal and a feedback signal.

20. A data converter comprising:

a time-to-voltage converter configured to generate an output voltage signal in response to an input interval and configured to generate a correlated reference voltage signal in response to a reference interval,

wherein the data converter is a time-to-digital converter or a digital-to-time converter,

wherein if the data converter is the time-to-digital converter, the data converter further comprises:

a phase detector configured to generate the input interval in response to a first input signal and a second input signal; and

an analog-to-digital converter configured to generate a digital code corresponding to the output voltage signal using the correlated reference voltage signal, and

wherein if the data converter is the digital-to-time converter, the data converter further comprises:

a digital-to-analog converter configured to use the correlated reference voltage signal to generate the input interval in response to an input digital code and an input time signal; and

a comparator configured to generate an output time signal in response to a comparison of the output voltage signal and a threshold voltage level of the comparator.